This invention relates to plasma processing of objects, and, more particularly, to plasma processing of electrically conducting or nonconducting objects.
The useful life of some types of objects is determined by the wear of the surface of the object. For example, the tooling used in metal forming operations such as stamping or forming must retain its close dimensional tolerances in order to be useful. When even a few thousandths of an inch of the surface is worn away, the tooling becomes ineffective and must be discarded or refurbished.
The cost of short runs of parts or pieces made with tooling is often high, due to the fixed cost of preparing acceptable tooling. One approach to reducing the tooling cost is to form the tool from a polymeric material such as an epoxy that is more readily made with the required shape and dimensions than is a comparable metal tool. The epoxy is much softer than metal, and can therefore be easily machined to the required shade and dimensional tolerances.
The surface softness that permits easy machining results in a short useful life of the tool. For production applications, an epoxy tool cannot be used without a treatment to make the surface more wear resistant. It has been found that several types of ion and electron treatments of the surfaces of polymers such as epoxies and the surfaces of metals can significantly increase their useful lives. Such treatments include, for example ion implantation, ion deposition and ion mixing, and electron bombardment.
Ion implantation is a process wherein ions are accelerated by an electrostatic potential to impact a surface. The energy of the ions causes them to be imbedded beneath the surface. A sufficient concentration of implanted ions can significantly increase the hardness of the surface layer. In ion mixing, a thin (about 300-600 Angstrom) material layer is first deposited onto a surface to be treated, and then the composite surface is ion implanted to mix the deposited material atoms into the surface. Ion deposition is similar to ion mixing except that the accelerating energies of the ions are lower, with the result that the ions are deposited simultaneously with the deposited material layer upon the surface being treated rather than imbedded beneath its surface. In electron bombardment of polymeric surfaces, energetic electrons (100-20,000 volts or more) are accelerated by an electrostatic potential to cause them to impact into the surface to be treated.
Such treatments are traditionally accomplished by accelerating a beam of electrical charge carriers (ions or electrons) using electrostatic acceleration electrodes. This approach, while effective, has the drawback that it is difficult to uniformly treat a large, three-dimensional, irregular object such as a typical automotive tool that may measure 3 feet by 5 feet by 1 foot in size. The beam of charge carriers must be moved slowly over the entire surface, including those areas that may be rather inaccessible such as deep holes or recesses, or for protrusions that stick out from the surface. In that case, the object must be manipulated to uniformly treat the area with a beam. For large tools or for large numbers of smaller objects having such surface features, surface treatment using ion or electron beams becomes prohibitively slow and expensive.
An alternative approach that has promise for ion implanting, ion depositing, or ion mixing of the surfaces of large objects is plasma ion implantation (PII), described in US Pat. No. 4,764,394. A plasma of ions is created adjacent to the surface of the object to be implanted, and the object is electrostatically charged to a potential opposite to that of the ions. For example, if positively charged nitrogen ions are to be implanted, the object is negatively charged using repetitive voltage pulses of typically about 100,000 volts or more. The nitrogen ions are attracted to the surface of the object by this accelerating potential and driven into the surface and sub-surface regions of the substrate. Plasma ion implantation has the advantage that the plasma of ions provides a source that is distributed around the entire surface area of the object, and uniform implantation over the entire surface area is simultaneously achieved. Plasma ion mixing may be performed in a similar manner, except that first a thin layer of material is deposited into the surface prior to implantation. Plasma ion deposition is also performed in a similar manner, except that the accelerating potential is typically lower so that the ions and material layer deposit upon the surface instead of being driven into the sub-surface layers. It is possible to achieve electron bombardment of a surface using the same plasma techniques used in ion implantation.
Plasma ion implantation, deposition and mixing, and electron bombardment work well for metallic objects with smooth, flat faces. However, where the object is electrically nonconducting, such as an epoxy, or is electrically conducting or nonconducting with sharp surface contours or deep recesses, the electric field distribution at the surface of the substrate is distorted and therefore nonuniform. The result is spatially nonuniform and uneven surface treatment, or a discontinuance of the surface treatment due to a space charge buildup with ensuing arcing. As an example, experiments by the inventors leading to the present invention have determined that in a plasma ion implantation system having a vacuum chamber diameter of 4 feet, some implantation can be achieved with relatively small polymeric epoxy objects, but that virtually no implantation can be achieved with epoxy objects that are four inches or more in thickness. Thus, conventional plasma ion implantation is ineffective for hardening the surface of large tooling pieces made of epoxy or other nonconducting materials. It is also ineffective for uniformly surface hardening either nonconducting or conducting objects that have sharp surface features or deep recesses.
There is a need for improved ion implantation, deposition and mixing, and electron bombardment techniques for use with electrically conducting or nonconducting objects, particularly such objects that are either relatively large in size or small in size but large in number, and such objects having sharp surface features or deep recesses. The present invention fulfills this need, and further provides related advantages.